Acrylonitrile-based synthetic fiber and method for production thereof

ABSTRACT

There is disclosed an acrylic fiber (a) consisting of an acrylonitrile polymer comprising an acrylonitrile unit in at least 80 wt % and less than 95 wt %, (b) having a monofilament dry strength of 2.5 to 4.0 cN/dtex, (c) having a monofilament dry elongation of 35 to 50%, and (d) forming a crack with a length of 20 μm or more in its tension rupture lateral surface along the filament axis direction when rupturing the monofilament in a tension test. The fiber has even orientation in its surface and inside; is significantly improved in dry strength, dry elongation and dyeability; and exhibits wool-like hand feeling. It is, therefore, quite suitable as a synthetic fiber for various applications such as a garment material, e.g., a sweater and a home furnishing material such as a pile.

TECHNICAL FIELD

This invention relates to an acrylic fiber generally suitable toapplications such as a garment and a home furnishing especially pilefabrics.

BACKGOUND ART

An acrylic fiber suitable to garments is required to have a good balancebetween its strength, elongation and dyeability.

An acrylic fiber is generally prepared by wet spinning. It has been aconventional practice to increase a ratio of (a drawing rate of acoagulated filament)/(a discharge linear velocity of a spinning feedsolution from a spinneret capillary) in a coagulation bath and toincrease a draw ratio in a subsequent step for achieving a high-strengthfiber with high orientation.

However, increasing a ratio of (a drawing rate of a coagulatedfilament)/(a discharge linear velocity of a spinning feed solution froma spinneret capillary) in a coagulation bath, i.e., increasing a drawingrate of a coagulated filament, means a shorter coagulation time for aspinning feed solution in the coagulation bath. Coagulation andstretching, therefore, simultaneously occur in the coagulation bath,resulting in formation of a skin layer in a coagulated filament, whichleads to inadequate solvent displacement inside the fiber.

Thus, the surface of the fiber has a higher fibrillated and highlyoriented structure, while its inside has a coarse structure withoutfibrillation. When stretched with a high stretching ratio, a productbecomes a fiber with a poor elongation, which will give a cloth with astiff hand feeling. A fiber with an uneven orientation between itssurface and inside provides a poorly elastic staple fiber, which willgive a cloth with an inadequate repulsion.

A fiber with an excessively oriented surface has a drawback of adeteriorated dyeability because the highly oriented surface inhibitsdiffusion of a dye during a dyeing process.

JP-A 61-199707 has described a spinning process using a coagulation bathwith a sufficiently higher concentration within a concentration rangethat a skin layer does not form. However, when using an aqueous solutionof an organic solvent as a coagulation bath, a concentration range ofthe organic solvent that a skin layer does not form is quite higher, sothat a coagulation rate becomes too late to increase a drawing rate ofthe coagulated filament, leading not only to an extremely lower yieldbut also to problems such as irregularities and fusion between fibers.

In home furnishing applications, particularly for a high-pile or boa, across section of a fiber is changed for providing hand feeling closer toanimal hair. In these applications, good brushing effect, higherflexibility, softness, etc. are required. Brushing effect is moreimproved as a friction on a fiber surface is lower. It is thus believedthat a dull material in which an additive such as titanium dioxide isused for emphasizing brightness generally exhibits an improved brushingeffect. In the technique, color-developing properties of an acrylicfiber are, however, hampered by the additive.

JP-A 11-21769 has disclosed a technique that apparent luster and fibercolor-developing are chosen as appropriate and an organopolysiloxane isbound to give slimy and smooth touch like an animal hair to the fibersurface. In the technique, while slimy and smooth touch is emphasized,the fiber may have poor softness and color-developing properties. It isnecessary for an acrylic fiber with reduced luster, goodcolor-developing properties and good brushing effect that its surface isnot smoothed but a contact area between fibers is reduced when it isprocessed to be a pile or boa cloth, by deliberately corrugating thefiber surface. For hand feeling, a fiber well-balanced in its strengthand elongation is required. In the light of these conditions, JP-A64-33210 has disclosed a process for preparing a dry acrylic fiber withmore natural luster by corrugating a fiber surface. In the process, aspinneret, however, has an orifice hole of special shape to corrugatethe surface. Thus, the fiber surface corrugation is considerablylimited.

Flexibility and softness in a boa or high pile may be achieved bycombining several types of fibers with different cross sections. It isbelieved that typically a flat or Y-shaped cross section of an acrylicfiber is effective for achieving the above properties. In particular, anacrylic fiber with a Y-shaped cross section gives soft hand feelingbecause its tip is split while having good flexibility because itretains a Y-shaped cross section in its root.

In the acrylic fiber disclosed in JP-A 10-251915, a monofilament 20 hasa substantially Y-shaped cross section where three radially extendingrectangular arms 21 are jointed with a jointing angle of 120° as shownin FIG. 7. In the joint of these arms 21, openings K1 or holes K2 areformed for adjusting the joint length c to be 30 to 95% of its width d.It allows the filament to be easily split along a longitudinal directionto realize soft hand feeling. In the acrylic fiber disclosed in thepatent application, a filament may be split before polisher processing aboa or high pile due to the openings K1 or the holes K2 formed in thejoint. Thus, it may result in, for example, generating fluffs duringspinning. Furthermore, the fiber may not be easily dried due to watertrapped in the openings K1 or the holes K2, leading to a longer dryingstep during spinning the fiber and thus to a reduced productivity.

DISCLOSURE OF THE INVENTION

An objective of this invention is to provide, for a garment material, anacrylic fiber which has even orientation in its surface and inside,gives a staple fiber with adequate elasticity to provide a cloth with arepulsion; and to provide the fiber which exhibits good physicalproperties such as a strength, an elongation and dyeability and exhibitssoftness by modifying its surface shape.

Another objective of this invention is to provide, for a home furnishingmaterial, an acrylic synthetic fiber which has good color-developingproperties with reduced luster and good brushing effect, and an acrylicsynthetic fiber which retains the status where a plurality of flat armsradially extending from a center along a longitudinal direction arejointed together and the fiber tip can be readily split by applying amechanical force during processing into a fluffy product.

Another objective of this invention is to provide a process for easilyand satisfactorily manufacturing an acrylic fiber which has evenorientation in its surface and inside and exhibits good properties suchas a strength, an elongation and dyeability, by, during preparing acoagulated filament, controlling the thickness of a skin layer of thecoagulated filament to provide a fiber evenly coagulated to its inside,i.e., by preventing a solvent inside the fiber from being inadequatelydiffused and thus preventing the solvent from being quickly diffusedduring washing.

The first aspect of this invention is directed to an acrylic fiber (a)consisting of an acrylonitrile polymer comprising an acrylonitrile unitin at least 80 wt % and less than 95 wt %, (b) having a monofilament drystrength of 2.5 to 4.0 cN/dtex, (c) having a monofilament dry elongationof 35 to 50%, and (d) forming a crack with a length of 20 μm or more inits tension rupture lateral surface along the filament axis directionwhen rupturing the monofilament in a tension test.

The second aspect of this invention is directed to an acrylic fiber (a)comprising corrugations on its surface, (b) having an average tilt angleof 15 to 20° between two adjacent corrugations in a cross sectionvertical to the fiber axis direction, (c) having a maximum leveldifference of 0.15 to 0.35 μm between the bottom and the top of thecorrugations, and (d) exhibiting a lusteriness of 10 to 20% in alusteriness determination method for a 45° mirror surface for a fiberbundle surface.

In one embodiment of the second aspect of this invention, the acrylicfiber (e) consists of an acrylonitrile polymer comprising anacrylonitrile unit in at least 80 wt % and less than 95 wt %, (f) has amonofilament dry strength of 2.0 to 4.0 cN/dtex, (g) has a monofilamentdry elongation of 15 to 40%, and (h) forms a crack with a length of 20μm or more in its tension rupture lateral surface along the filamentaxis direction when rupturing the monofilament in a tension test.

The third aspect of this invention is directed to an acrylic fiber (a)comprising a plurality of flat arms radially extending from a centeralong a longitudinal direction and (b) forming a crack with a length of200 μm or more in the center of its tension rupture lateral surfacealong the filament axis direction when rupturing the monofilament in atension test.

In one embodiment of the third aspect of this invention, the acrylicfiber (c) consists of an acrylonitrile polymer comprising anacrylonitrile unit in at least 80 wt % and less than 95 wt %, (d) has amonofilament dry strength of 2.0 to 4.0 cN/dtex, and (e) has amonofilament dry elongation of 15 to 40%.

This invention further provides a process for manufacturing an acrylicfiber comprising the steps of: discharging a spinning feed solutioncomprising an acrylonitrile polymer comprising 80 wt % or more and lessthan 95 wt % of acrylonitrile unit in an organic solvent, into the firstcoagulation bath consisting of an aqueous organic solvent solution at 30to 50° C. containing 20 to 70 wt % of an organic solvent which may bethe same as or different from the organic solvent for the spinning feedsolution, to form a coagulated filament; drawing the filament from thefirst coagulation bath at a rate of 0.3 to 2.0 times of the dischargelinear velocity of the spinning feed solution; stretching the filamentby 1.1 to 2.0 times in the second coagulation bath consisting of anaqueous organic solvent solution at 30 to 50° C. containing 20 to 70 wt% of an organic solvent which may be the same as or different from anyof the two organic solvents; and subsequently conducting wet heatstretching of the filament by three times or more.

In one embodiment of the above manufacturing process, there is provideda process where the concentration of the organic solvent in the firstcoagulation bath is 40 to 70 wt %; the drawing rate of a coagulatedfilament from the first coagulation bath is 0.3 to 0.6 times of thedischarge linear velocity of the spinning feed solution; and theconcentration of the organic solvent in the second coagulation bath is40 to 70 wt %.

In another embodiment of the above manufacturing process, there isprovided a process where the concentration of the organic solvent in thefirst coagulation bath is 20 to 60 wt %; the drawing rate of acoagulated filament from the first coagulation bath is 0.6 to 2.0 timesof the discharge linear velocity of the spinning feed solution; and theconcentration of the organic solvent in the second coagulation bath is20 to 60 wt %.

It is preferable in the manufacturing processes of this invention thatthe organic solvents in the spinning feed solution, the firstcoagulation bath and the second coagulation bath are dimethylacetamideand the first and the second coagulation bathes are at the sametemperature and have the same composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph on the xy plane illustrating the straight linesrepresented by the following equations:

Y=−X+105  (Eq.1)

Y=−(½)X+77.5  (Eq.2)

Y=−4X+315  (Eq.3)

wherein Y is a coagulation-bath temperature (° C.) and X is aconcentration of an organic solvent (wt %).

FIG. 2 schematically shows the status of a crack part formed in atension rupture lateral surface of a monofilament in a tension test asobserved by scanning electron microscopy, in which the crack isrelatively long.

FIG. 3 schematically shows the status of a crack part formed in atension rupture lateral surface of a monofilament in a tension test asobserved by scanning electron microscopy, in which the crack isrelatively short.

FIG. 4 is a conceptual diagram illustrating a part of a fiber surfaceshape, where (a) is a tilt angle (an average tilt angle is determined bymeasuring a tilt angle for each corrugation and then averaging them) and(b) is a level difference (a maximum level difference is the differencebetween the higher and the lower parts).

FIG. 5(a) is a conceptual diagram for determination of a luster, andFIG. 5(b) shows a sample model when determining a luster.

FIG. 6 is a front view illustrating an example of the shape of aspinneret capillary in a spinneret used in a process for manufacturingan acrylic fiber according to this invention.

FIG. 7 schematically shows a cross section of a conventional acrylicfiber.

FIG. 8(a) is a scanning electron microscope (SEM) photograph which showsoblique view of the fiber obtained in example 1. FIG. 8(b) is a SEMphotograph which shows a lateral surface of the fiber obtained inexample 1 which was ruptured in the tension test.

FIG. 9(a) is a SEM photograph which shows oblique view of the fiberobtained in comparative example 1. FIG. 9(b) is a SEM photograph whichshows a lateral surface of the fiber obtained in comparative example 1which was ruptured in the tension test.

FIG. 10 is a SEM photograph which shows oblique view of the fiberobtained in example 3.

FIG. 11 is a SEM photograph which shows oblique view of the fiberobtained in comparative example 5.

FIG. 12(a) is a SEM photograph which shows oblique view of the fiberobtained in example 7. FIG. 12(b) is a SEM photograph which shows thesurface of the fiber obtained in example 7.

FIG. 13(a) is a SEM photograph which shows oblique view of the fiberobtained in comparative example 6. FIG. 13(b) is a SEM photograph whichshows the surface of the fiber obtained in comparative example 6.

FIG. 14(a) is a SEM photograph which shows oblique view of the fiberobtained in example 9. FIG. 14(b) is a SEM photograph which shows alateral surface of the fiber obtained in example 9 which was ruptured inthe tension test.

FIG. 15(a) is a SEM photograph which shows oblique view of the fiberobtained in comparative example 11. FIG. 15(b) is a SEM photograph whichshows a lateral surface of the fiber obtained in comparative example 11which was ruptured in the tension test.

BEST MODE FOR CARRYING THE INVENTION

An acrylic fiber of this invention is suitable mainly to a garment suchas a sweater and a home furnishing application such as a pile. In thelight of solubility of a polymer and stability of a spinning feedsolution during fibrillation by wet spinning, it is preferable to use acopolymer with a relatively small amount of acrylonitrile unit, i.e.,less than 95 wt % of acrylonitrile, as a fiber material. If the amountof acrylonitrile unit is too low in the acrylonitrile polymer used as afiber material, there may be inadequate wool-like hand feeling requiredfor an acrylic fiber for an application such as sweater and a pileproduct. The concentration is, therefore, preferably at least 80 wt %.

The material may be a mixture of acrylonitrile polymers containing atleast 80 wt % and less than 95 wt % of acrylonitrile.

An acrylonitrile polymer is a copolymer of acrylonitrile with a monomerpolymerizable with acrylonitrile. Monomers which may be used as acopolymer component include, but not limited to, (meth)acrylates such asmethyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate,butyl (meth)acrylate and hexyl (meth)acrylate; vinyl halides such asvinyl chloride, vinyl bromide and vinylidene chloride; acids having apolymerizable double bond and their salts such as (meth)acrylic acid,itaconic acid and crotonic acid; maleimide; phenylmaleimide;(meth)acrylamide; styrene; α-methylstyrene; vinyl acetate;sulfone-containing polymerizable unsaturated monomers such as sodiumstyrenesulfonate, sodium allylsulfonate, β-sodium styrenesulfonate,sodium methallylsulfonate; and pyridine-containing polymerizableunsaturated monomers such as 2-vinylpyridine and2-methyl-5-vinylpyridine.

An acrylonitrile polymer as a fiber material may be readily prepared by,for example, redox polymerization using an aqueous solution, suspensionpolymerization in a heterogeneous system, emulsion polymerization usinga dispersing agent or any other polymerization method.

An acrylic fiber in the first embodiment of this invention has amonofilament dry strength of 2.5 to 4.0 cN/dtex, has a monofilament dryelongation of 35 to 50%, and forms a crack with a length of 20 μm ormore in its tension rupture lateral surface along the filament axisdirection when rupturing the monofilament in a tension test.

If the monofilament dry strength is lower than 2.5 cN/dtex or the dryelongation is more than 50% in the acrylic fiber, there may be generatedmany fluffs due to filament breaking during spinning, leading to adeteriorated process passage and significant deterioration inspinnability.

If the monofilament dry strength is higher than 4.0 cN/dtex or the dryelongation is less than 35%, there may often be inadequate wool-likehand feeling required for an acrylic fiber for an application such as agarment, e.g., a sweater and a home furnishing, e.g., a pile.

The length of the crack formed along a fiber axis in a tension test isan index indicating difference in orientation between the surface andthe inside of the fiber. The feature of a crack with a length of 20 μmor more in the tension rupture lateral surface of the monofilament alongthe filament axis direction in the acrylic fiber of this inventionindicates a structure in which orientation is even not only in itssurface layer but also in its inside.

FIG. 2 shows a ruptured acrylic fiber in which orientation is even notonly in its surface layer but also in its inside, in a tension test. Theacrylic fiber evenly oriented to its inside, i.e., orientation is evenboth in its surface and its inside, is ruptured in a tension rupturetest such that there are a plurality of rupture points in a tensionrupture section. There are, therefore, formed a long crack in thetension rupture lateral surface along the fiber axis direction. It ispredicted that the fiber has a structure evenly oriented not only in itssurface layer but also in its inside if the length L from the bottom Bto the top S of the crack is 20 μm or more as shown in FIG. 2.

On the other hand, FIG. 3 shows a ruptured acrylic fiber in which itssurface is oriented while its inside is of a coarse structure, in atension test. Such a fiber is ruptured in a tension rupture test suchthat there is one rupture point in a tension rupture section. There isnot, therefore, formed a crack in the tension rupture lateral surface ofthe monofilament along the fiber axis direction, or if any, it is quiteshort. The length L from the bottom B to the top S of the crack is lessthan 20 μm as shown in FIG. 3. A staple fiber made of the fiber has aninadequate elasticity. As a result, a cloth after processing does nothave an adequate repulsion and thus does not exhibit satisfactory handfeeling required for a cloth utilized in an application such as agarment, e.g., a sweater and a home furnishing, e,g., a pile.

A status of the tension rupture lateral surface of a monofilament isobserved for a rupture surface formed after rupturing the monofilamentat a deformation rate of 100%/min under the conditions of 23° C. and 50%RH.

In an acrylic fiber according to the first aspect of this invention, afiber cross section is preferably a perfect or essentially perfectcircle in the light of spinnability, color-developing properties andwool-like elasticity. Specifically, a ratio of long/short axes in thefiber cross section is preferably 1.0 to 2.0, more preferably 1.0 to 1.2which means a more perfect circle. A fiber having such a cross sectionis suitable to a garment such as a sweater.

Next, there will be described an acrylic fiber according to the secondaspect of this invention.

The acrylic fiber of the second aspect of this invention has finecorrugations on its surface which may be observed as creases. In thecrease-like corrugations, an average tilt angle between adjacentcorrugations (hereinafter, referred to as an “average tilt angle”) is 15to 20° in a cross section perpendicular to the fiber axis direction anda maximum level difference between the bottom and the top of thecorrugations (a maximum level difference between the bottom and the topof the creases; hereinafter, referred to as a “maximum leveldifference”) is 0.15 to 0.35 μm.

When an acrylic fiber meets the conditions of an average tilt angle of15 to 20° and a maximum level difference of 0.15 to 0.35 μm, a contactarea between fibers is reduced, brushing effect is improved, softness isprovided after processing into a pile or boa, and the surfacecorrugations control luster in the fiber. If the average tilt angle isless than 15°, the number of corrugations or the creases is increased,and may lead to increase in a contact area between fibers and thus todeteriorated brushing effect. If the average tilt angle is higher than25°, the corrugations or the creases are reduced, so that a contact areabetween fibers is increased.

If the maximum level difference is less than 0.15 μm, brushing effect(i.e. hair handle property) tends to be poor and slimy and smooth touchwhich adversely affects hand feeling may be caused. On the other hand,if more than 0.35 μm, the fiber may be readily split, leading toproblems in processability such as spinnability.

An acrylic fiber according to the second aspect of this inventionexhibits (d) a luster of 10 to 20% in a luster determination method fora 45° mirror surface for a fiber bundle surface. Tone after processinginto a pile or boa may be less deep when a luster is too high, whilecolor developing is reduced when a luster is too low. Thus, the aboverange is preferable.

Preferably, the acrylic fiber according to the second aspect of thisinvention further (e) consists of an acrylonitrile polymer comprising anacrylonitrile unit in at least 80 wt % and less than 95 wt %, (f) has amonofilament dry strength of 2.0 to 4.0 cN/dtex, (g) has a monofilamentdry elongation of 15 to 40%, and (h) may form a crack with a length of20 μm or more in its tension rupture lateral surface along the filamentaxis direction when rupturing the monofilament in a tension test.

In the second aspect of this invention, if the monofilament dry strengthof the acrylic fiber is less than 2.0 cN/dtex or its dry elongation ismore than 40%, there may be generated many fluffs due to filamentbreaking during spinning, leading to a deteriorated process passage andpoor hand feeling due to elongation of the fiber during boa or high-pileprocessing.

If the monofilament dry strength is higher than 4.0 cN/dtex or the dryelongation is less than 15%, there may often be inadequate wool-likehand feeling required for an acrylic fiber for an application such as agarment, e.g., a sweater and a home furnishing, e.g., a pile.

As mentioned above, the feature of a crack with a length of 20 μm ormore in the tension rupture lateral surface of the monofilament alongthe filament axis direction indicates a structure in which orientationis even not only in its surface layer but also in its inside. Therefore,after processing, it provides a cloth with an adequate repulsion meetinghand feeling required for a cloth for a garment such as a sweater and ahome furnishing such as a pile.

In the acrylic fiber of the second aspect of this invention, for a homefurnishing material such as a pile and a boa, the long/short axis ratioin its cross section (flatness) is preferably 5 to 15 in the light ofhand feeling and flexibility after being processed into a pile or boa.Flexibility is not adequate if the flatness is less than 5 afterprocessed into a pile of boa, while the fiber tends to be split,causing, for example, irritation if it is more than 15.

There will be described an acrylic fiber according to the third aspectof this invention.

The acrylic fiber of this aspect comprises a plurality of flat armsradially extending from a monofilament center along a longitudinaldirection. In other words, the cross section of the monofilament has abranched shape radially extending from the center such as an essentiallyY-shape or cross shape. An angle formed by adjacent flat arms may be thesame or different. For example, in an essentially Y-shape, three flatarms may be mutually extended at an angle of 120°. The cross section(the length in the axis direction and the width) of each flat armconstituting a monofilament may be mutually the same or different.Different cross sections may endow various additional hand feeling.

A monofilament comprising a plurality of flat arms radially extendingfrom a monofilament center along a longitudinal direction may provide,after processing, a fluffy product with satisfactory softness andflexibility. In particular, the filament cross section is preferably anessentially Y-shape or cross shape with three or four flat arms forensuring adequate flexibility in its root when its tip is split.Increase in the number of the arms may cause problems in manufacturing aspinneret and in manufacturing a fiber such as trapped water in the armroot adversely affecting drying and reduced spinnability . Themonofilament most preferably has an essentially Y-shape consisting ofthree flat arms.

The acrylic fiber of the third aspect forms a crack with a length of 200μm or more in the center of its tension rupture lateral surface alongthe filament axis direction when rupturing the monofilament in a tensiontest. Again, a status of the tension rupture lateral surface of amonofilament is observed for a rupture surface formed after rupturingthe monofilament at a deformation rate of 100%/min under the conditionsof 23° C. and 50% RH.

In this aspect, the feature of forming a long crack in the tensionrupture lateral surface of the monofilament along the filament axisdirection again indicates a structure in which orientation is even notonly in its surface layer but also in its inside. However, the fiber ofthe third aspect has flat arms and tends to be split from its center. Acrack with a length of at least 20 μm is, therefore, not adequate, but acrack of at least 200 μm from its center must be formed.

Such an acrylic fiber exhibits good softness because monofilament tipsare split to an adequate length while it can retain adequate flexibilitywithout split in a filament root. Excessively larger split may improvesoftness but reduce flexibility and does not give required hand feeling.Therefore, the crack length formed in the tension test is preferablyless than 1000 μm.

The acrylic fiber of the third aspect preferably (c) consists of anacrylonitrile polymer comprising an acrylonitrile unit in at least 80wt% and less than 95 wt %, (d) has a monofilament strength of 2.0 to 4.0cN/dtex, and (e) has a monofilament elongation of 15 to 40%.

In the third aspect, if the monofilament dry strength of the dry acrylicfiber is less than 2.0 cN/dtex or its dry elongation is more than 40%,there may be generated many fluffs due to filament breaking duringspinning, leading to a deteriorated process passage and significantdeterioration in tip split property due to dry elongation of the fiberduring polisher processing in boa or high pile formation.

If the monofilament dry strength is higher than 4.0 cN/dtex or the dryelongation is less than 15%, there may often be inadequate wool-likehand feeling required for an acrylic fiber for an application such as agarment, e.g., a sweater and a home furnishing, e.g., a pile.

In the acrylic fiber of the third aspect, a Young's modulus ispreferably 5800 N/mm² or higher because a too low Young's modulus maygive inadequate repulsion of a cloth after processing into a pile,leading to a poorly flexible product. In the light of hand feeling inthe pile, the Young's modulus is more preferably 7000 to 12000 N/mm² forachieving both flexibility and softness.

Furthermore, in the acrylic fiber of the third aspect, a ratio of a/b ispreferably 2.0 to 10.0, where “a” and “b” are the monofilament lengthfrom its center to the tip of the flat arm and the width of the flatarm, respectively. A too low ratio a/b may lead to inadequateflexibility while a too high ratio may cause excessive flexibility sothat even split filament tips cannot provide adequate softness.

Next, there will be described a manufacturing process according to thisinvention.

A process for manufacturing an acrylic fiber comprises the steps ofdischarging a spinning feed solution comprising an acrylonitrile polymercomprising 80 wt % or more and less than 95 wt % of acrylonitrile unitin an organic solvent, into the first coagulation bath consisting of anaqueous organic solvent solution at 30 to 50° C. containing 20 to 70 wt% of an organic solvent which may be the same as or different from theorganic solvent for the spinning feed solution, to form a coagulatedfilament; drawing the filament from the first coagulation bath at a rateof 0.3 to 2.0 times of the discharge linear velocity of the spinningfeed solution; stretching the filament by 1.1 to 2.0 times in the secondcoagulation bath consisting of an aqueous organic solvent solution at 30to 50° C. containing 20 to 70 wt % of an organic solvent which may bethe same as or different from any of the two organic solvents; andsubsequently conducting wet heat stretching of the filament by threetimes or more.

Organic solvents which may be used in the manufacturing process of thisinvention can dissolve an acrylonitrile polymer; for example,dimethylacetamide, dimethylsulfoxide and dimethylformamide.Dimethylacetamide is particularly preferable because it is not affectedby hydrolysis and exhibits good spinnability.

The conditions for the first coagulation bath, the conditions for thesecond coagulation bath and stretching in the second coagulation bathare important for improving orientation in an acrylic fiber produced.

It is preferable for even coagulation during forming a coagulatedfilament that both coagulation bathes have the essentially same organicsolvent concentration. Specifically, a difference in an organic solventconcentration between these coagulation bathes is within 5 wt %,preferably within 3 wt %.

It is also preferable for even coagulation during forming a coagulatedfilament that both coagulation bathes are kept at the substantially sametemperature. A temperature difference between the first and the secondcoagulation bathes is within 5° C., more preferably within 3° C.

It is also preferable that these bathes comprise the same organicsolvent. It is particularly preferable that the spinning feed solution,the first coagulation bath and the second coagulation bath comprise thesame organic solvent, for even coagulation during forming a coagulatedfilament, easy preparation of these coagulation bathes and easy recoveryof the solvent.

Thus, most preferably, the spinning feed solution, the first coagulationbath and the second coagulation bath comprise dimethylacetamide as anorganic solvent. It is particularly preferable to use dimethylacetamideas an organic solvent for these three solutions and to use an aqueousdimethylacetamide solution at the substantially same temperature andhaving the substantially same composition in the first and the secondcoagulation bathes.

In the process for manufacturing an acrylic fiber according to thisinvention, a coagulated filament drawn from the first coagulation bathis in a semi-coagulated state where only its surface is coagulated sincethe organic solvent concentration in the liquid contained in thecoagulated filament is higher than that in the first coagulation bath.The filament can be, therefore, well stretched in the next step. Theswollen coagulated filament containing the coagulation solution afterdrawing it from the first coagulation bath may be stretched in the air,but it is preferably stretched in the second coagulation bath foraccelerating coagulation of the coagulated filament and easilycontrolling a temperature in the stretching step.

A draw ratio less than 1.1 in the second coagulation bath may fail togive an evenly oriented filament while a draw ratio higher than 2.0tends to cause filament breaking, leading to reduced spinnability anddeteriorated stretching properties during the subsequent wet heatstretching step.

In one embodiment of the manufacturing process of this invention, it ispreferable that the concentration of the organic solvent in the firstcoagulation bath is 40 to 70 wt %; the drawing rate of a coagulatedfilament from the first coagulation bath is 0.3 to 0.6 times of thedischarge linear velocity of the spinning feed solution; and theconcentration of the organic solvent in the second coagulation bath is40 to 70 wt %. Among these conditions, the drawing rate of a coagulatedfilament from the first coagulation bath is particularly characteristic.It may allow the thickness of the skin layer in the coagulated filamentdrawn from the first coagulation bath to be adjusted to 0.05 to 0.15 μm.The skin layer thinner than 0.05 μm in the coagulated filament drawnfrom the first coagulation bath tends to cause adhesion of filaments andirregular coagulation in the coagulation bath, leading to a fiber withpoor cotton properties, while the skin layer thicker than 0.15 μm mayinhibit coagulation of the coagulated filament and make the inside ofthe filament coarse, leading to a fiber whose surface is more oriented.

In the process of this invention, it is preferable that the first andthe second coagulation bathes are at the same temperature and have thesame composition, and that a coordinate (X,Y) is within the areadelimited by the lines represented by the following equations (1) to(3):

Y=−X+105  (Eq.1)

Y=−(½)X+77.5  (Eq.2)

Y=−4X+315  (Eq.3)

wherein Y is the coagulation-bath temperature (° C.) and X is theconcentration of the organic solvent (wt %).

The area delimited by these three lines is the triangle on the xy planein FIG. 1. The coordinate (X,Y) within the triangle may allow asynthetic fiber with a perfect or substantially perfect circle crosssection to be further exactly prepared, and therefore, make the processof this invention suitable to manufacturing an acrylic fiber for acloth. It is particularly preferable that the drawing rate of acoagulated filament from the first coagulation bath is 0.3 to 0.6 timesof the discharge linear velocity of the spinning feed solution.

In another aspect of the manufacturing process of this invention, it ispreferable that the concentration of the organic solvent in the firstcoagulation bath is 20 to 60 wt %; the drawing rate of a coagulatedfilament from the first coagulation bath is 0.6 to 2.0 times of thedischarge linear velocity of the spinning feed solution; and theconcentration of the organic solvent in the second coagulation bath is20 to 60 wt %. Among these conditions, the drawing rate of a coagulatedfilament from the first coagulation bath is again particularlycharacteristic. A higher drawing rate of a coagulated filament resultsin quick coagulation. Thus, the process is suitable to manufacturing afiber with branched flat arms such as an essentially Y-shaped structureor a flat fiber which requires a sharp cross section.

For forming a fiber with flat arms radially branched from the center ofa monofilament (typically, an essentially Y-shaped or cross-shapedstructure), it is preferable that a spinneret capillary in a spinnerethas such a shape. For example, it is preferable to use a spinneret witha spinneret capillary where a ratio A/B is 2.0 to 10.0 wherein “A” and“B” are the length of each radially branched opening arm from its centerand the width of the branched opening arm, respectively.

When forming a flat fiber with a large ratio of long/short axes(flatness) in the fiber cross section, it is preferable to use aspinneret with a spinneret capillary in which a long/short axis ratio(flatness) is 5.0 to 15.0.

In the manufacturing processes of this invention, after stretching inthe second coagulation bath, wet heat stretching of the filament bythree times or more is conducted for further improving orientation in afiber. Wet heat stretching may be conducted by stretching a swollenfiber just after stretching in the second coagulation bath while washingit with water, or by stretching it in hot water. For improving aproductivity, stretching in hot water is preferable. More preferably,the fiber is stretched while washing with water, and subsequentlystretched in hot water. If the stretching ratio in the wet heatstretching is less than 3, fiber orientation may be inadequatelyimproved. The stretching ratio in the wet heat stretching may beappropriately selected as long as it is more than 3, but it is generallyabout 8 or less.

The fiber after stretching in the second coagulation bath may be driedbefore stretching. Stretching after drying may, however, often generatestatic electricity which considerably deteriorates convergency of thefilaments. On the other hand, significant deterioration in convergencyassociated with stretching can be avoided according to the process ofthis invention where wet heat stretching is employed after stretching inthe second coagulation bath.

In the process for manufacturing an acrylic fiber of this invention, itis preferable to adjust a degree of swelling of the swollen fiber afterwet heat stretching and before drying to 70 wt % or less.

A swollen fiber after wet heat stretching and before drying whose degreeof swelling is 70 wt % or less indicates that orientation is even inboth its surface and inside. By reducing the ratio of (a drawing rate ofa coagulated filament)/(a discharge linear velocity of a spinning feedsolution from a spinneret capillary) during preparing a coagulatedfilament in the first coagulation bath, formed is the coagulatedfilament even in the first coagulation bath. Then the filament may bestretched in the second coagulation bath to prepare a fiber whoseorientation is even to its inside. Thus, a degree of swelling of theswollen fiber after wet heat stretching and before drying can be adjustto 70 wt % or less.

In other words, when the ratio of (a drawing rate of a coagulatedfilament)/(a discharge linear velocity of a spinning feed solution froma spinneret capillary) is increased during preparing a coagulatedfilament in the first coagulation bath, coagulation of the coagulatedfilament occurs simultaneously with its stretching in the firstcoagulation bath, so that the coagulated filament is unevenly coagulatedin the first coagulation bath. Therefore, even if the stretching in thesecond coagulation bath is performed, a degree of swelling of a swollenfiber after wet heat stretching and before drying is high. This meansorientation of the resulting fiber is not even to its inside.

A degree of swelling of a swollen fiber before drying is calculated fromthe following equation:

A degree of swelling (%)=(w−w₀)×100/w₀

wherein “w” is a fiber weight after removing adhered liquid to theswollen fiber by centrifugation (3000 rpm, 15 min) and “w₀” is a fiberweight after drying the centrifuged fiber in a hot air dryer at 110° C.for 2 hours.

As described above, a fiber after stretching in the second coagulationbath and subsequent wet heat stretching is dried by a known process toprepare a desired acrylic fiber.

There will be specifically described an acrylic fiber according to thisinvention and a manufacturing process therefor with reference toExamples.

Tension Rupture Test

Using Tensilon UTM-II, a test monofilament with a length of 20 mm wasruptured with a deformation velocity of 100%/min under the conditions of23° C. and 50% RH to prepare a test sample. The outer surface of thetest sample was adhered to a sample plate for SEM and then the samplewas subject to spattering with Au to about 10 nm. The sample wasobserved with an XL 20 scanning electron microscope (PHILIPS) under theconditions: an acceleration voltage of 7.00 kV and a working distance of31 mm.

Determination of a Long/short Axis Ratio of a Fiber Cross Section, aLength of a Flat Arm to its Tip “a” and its Width “b”

A long/short axis ratio of a fiber cross section was determined byinserting an acrylic fiber to be measured into a vinyl chloride resintube with an inner diameter of 1 mm, cutting it into rings with a knifeto prepare a test sample, adhering the test sample to a sample plate forSEM such that the cross section of the acrylic fiber faces upward,spattering the sample with Au to about 10 nm and then observing thesample with an XL 20 scanning electron microscope (PHILIPS) under theconditions: an acceleration voltage of 7.00 kV and a working distance of31 mm. A length of a flat arm to its tip “a” and its width “b” aredetermined in the same manner.

Determination of an Average Tilt Angle and a Maximum Level Difference

A fiber is fixed on a slide glass using a double sided adhesive tapewithout tension, and observed by using a small-sized bench type of probemicroscope Nanopics (Seiko Instruments Inc.). An average tilt angle anda maximum level difference are determined as follows. As shown in FIG.4, the fiber surface is expressed as a wave form where selecting a linepassing corrugation trough bottoms as a base line, an ordinate and anabscissa are a corrugation height and its length along the fiberperiphery, respectively. Along the abscissa, perpendicular lines aredrawn with a fine interval (0.015 μm interval), intersections of theperpendicular lines with the wave form are connected, and all of angles(a) less than 90° formed by the line and the perpendicular line areaveraged to give an average tilt angle. A difference between the highestconvex and the lowest concave (b) is a maximum level difference.

Measurement Conditions

Measurement mode: Damping mode

observation range: 4 μm

Scanning rate: 90 sec/frame

Datum point number per an image: 512 pixel×256 lines

Determination of Luster in a Fiber Bundle by 45° Mirror Surface LusterTechnique

As shown in FIGS. 5(a) and 5(b), a fiber bundle (spinning tow) 3 with atotal denier of 150 to 200 d was tightly wound on an acrylic resin plate4 with a width of 50 mm and a thickness of 3 mm, without overlapping toprepare a sample with a width of 40 mm. Using VGS-300A (NIPPONDENSHOKU), an incident direction of light beam from a light source 1 wasadjusted to vertical to the fiber axis of the sample. Furthermore,adjusting an incident angle of light beam from the light source 1 and areceiving angle at a receiver 2 to 45° to a perpendicular line,respectively, a luster was determined by a 45° mirror surface lustertechnique in accordance with JIS-Z-8741.

Determination of a Thickness of a Skin Layer in a Coagulated Filament

A coagulated filament drawn from the first coagulation bath was soakedin an aqueous organic solvent solution having the same composition asthe first coagulation bath. Then, the filament was sequentially soakedat room temperature in mixtures of an aqueous organic solventsolution/ethanol with the ratio of “the aqueous organic solventsolution/ethanol” being gradually changed. The solution was finallyreplaced with ethanol. The filament was sequentially soaked in mixtureof ethanol/Spurr Resin (an epoxy resin for embedding a electronmicroscopy sample) with gradually changing the ratio, and Spurr Resin(i.e., replacement with Spurr Resin). Then, the filament was leftovernight to be subject to polymerization embedding to prepare a sample.The sample was cut into rings with a microtome, one of which was thenobserved with a transmission electron microscope at an accelerationvoltage of 40 kV to determine the thickness of the skin layer in thecoagulated filament.

EXAMPLE 1

A monomer composition consisting of 92 wt % of acrylonitrile and 8 wt %of vinyl acetate was polymerized by aqueous dispersion polymerizationusing ammonium persulfate-sodium hydrogen sulfite to prepare anacrylonitrile polymer with an average molecular weight of 130,000. Thepolymer was dissolved in dimethylacetamide to prepare a 24 wt % spinningfeed solution.

The spinning feed solution was discharged into the first coagulationbath consisting of a 50 wt % aqueous dimethylacetamide solution at 40°C. using a spinneret with 40,000 orifice holes and an orifice holediameter of 60 μm to prepare coagulated filaments. The filaments weredrawn from the first coagulation bath with a drawing rate 0.4 times ofthe discharge linear velocity of the spinning feed solution. Then, thecoagulated filaments were immersed into the second coagulation bathconsisting of a 50 wt % aqueous dimethylacetamide solution at 40° andwas subject to stretching by 1.5 times in the bath. While washing withwater, the filaments were further stretched by 2.7 times and in hotwater by 1.9 times. Then, the filaments were oiled, dried on a hot rollat 150° C., crimped, heated and cut to provide a staple fiber with amonofilament denier of 3.3 dtex.

In the above process, a monofilament cross section of the coagulatedfilaments drawn from the first coagulation bath was observed with atransmission electron microscope. The thickness of the skin layer was0.1 μm. The monofilament exhibited a dry strength of 3.2 cN/dtex, a dryelongation of 45%, and the staple fiber exhibited good luster and handfeeling.

The observation using scanning electron microscopy was conducted for amonofilament cross section and a tension rupture lateral surface of amonofilament. The filament cross section was an ellipse with along/short axis ratio of 1.8. Four cracks with lengths of 25 μm, 20 μm,20 μm and 18 μm along the fiber axis direction were observed in thetension rupture lateral surface.

EXAMPLE 2

A staple fiber with a monofilament denier of 3.3 dtex was prepared asdescribed in Example 1, except that the temperatures of the first andthe second coagulation bathes were 46° C. and the concentration of theorganic solvent was 60 wt %.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.08 μm. Themonofilament exhibited a dry strength of 3.5 cN/dtex, a dry elongationof 37%, and the staple fiber exhibited good luster and hand feeling.

The filament cross section was an essentially perfect circle with along/short axis ratio of 1.1. Five cracks with lengths of 25 μm, 24 μm,20 μm, 18 μm and 15 μm along the fiber axis direction were observed inthe tension rupture lateral surface.

EXAMPLE 3

The spinning feed solution described in Example 1 was discharged intothe first coagulation bath consisting of a 67 wt % aqueousdimethylacetamide solution at 40° C. using a spinneret with 40,000orifice holes and an orifice hole diameter of 60 μm to preparecoagulated filaments. The filaments were drawn from the firstcoagulation bath with a drawing rate 0.3 times of the discharge linearvelocity of the spinning feed solution. Then, the coagulated filamentswere immersed into the second coagulation bath consisting of a 67 wt %aqueous dimethylacetamide solution at 40° and was subject to stretchingby 1.5times in the bath. While washing with water, the filaments werefurther stretched by 2.7times and in hot water by 1.9 times. Then, thefilaments were oiled, dried on a hot roll at 150° C., crimped, heatedand cut to provide a staple fiber with a monofilament thickness of 2.2dtex.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.07 μm. Themonofilament exhibited a dry strength of 3.4 cN/dtex, a dry elongationof 40%, and the staple fiber exhibited good luster and hand feeling.

The filament cross section was an essentially perfect circle with along/short axis ratio of 1.05. Six cracks with lengths of 30 μm, 26 μm,22 μm, 21 μm, 18 μm and 15 μtm along the fiber axis direction wereobserved in the tension rupture lateral surface.

EXAMPLE 4

A staple fiber with a monofilament denier of 2.2 dtex was prepared asdescribed in Example 3, except that the temperatures of the first andthe second coagulation bathes were 46° C. and the concentration of theorganic solvent was 60 wt %.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.09 μm. Themonofilament exhibited a dry strength of 2.9 cN/dtex, a dry elongationof 37%, and the staple fiber exhibited good luster and hand feeling.

The filament cross section was an essentially perfect circle with along/short axis ratio of 1.1. Three cracks with lengths of 26 μm, 24 μmand 21 μm along the fiber axis direction were observed in the tensionrupture lateral surface.

EXAMPLE 5

A staple fiber with a monofilament denier of 2.2 dtex was prepared asdescribed in Example 3, except that the temperatures of the first andthe second coagulation bathes were 45° C. and the concentration of theorganic solvent was 58 wt %.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was μ0.1 m. Themonofilament exhibited a dry strength of 2.8 cN/dtex, a dry elongationof 37%, and the staple fiber exhibited good luster and hand feeling.

The filament cross section was an essentially perfect circle with along/short axis ratio of 1.2. Two cracks with lengths of 25 μm and 20 μmalong the fiber axis direction were observed in the tension rupturelateral surface.

EXAMPLE 6

A staple fiber with a monofilament denier of 2.2 dtex was prepared asdescribed in Example 3, except that the temperatures of the first andthe second coagulation bathes were 38° C. and the concentration of theorganic solvent was 65 wt %.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.06 μm. Themonofilament exhibited a dry strength of 3.3 cN/dtex, a dry elongationof 39%, and the staple fiber exhibited good luster and hand feeling.

The filament cross section was an essentially perfect circle with along/short axis ratio of 1.15. Five cracks with lengths of 31 μm, 27 μm,23 μm, 20 μm and 18 μm along the fiber axis direction were observed inthe tension rupture lateral surface.

EXAMPLE 7

A monomer composition consisting of 92 wt % of acrylonitrile and 8 wt %of vinyl acetate was polymerized by aqueous dispersion polymerizationusing ammonium persulfate—sodium hydrogen sulfite to prepare a polymerwith an average molecular weight of 130,000. The polymer was dissolvedin dimethylacetamide to prepare a 24 wt % spinning feed solution.

The spinning feed solution was discharged into the first coagulationbath consisting of a 30 wt % aqueous dimethylacetamide solution at 40°C. using a spinneret with 10,000 orifice holes and an orifice hole sizeof 0.035 mm×0.3 mm under the condition of a ratio of “a drawing rate ofa coagulated filament/a discharge linear velocity of a spinning feedsolution from a spinneret capillary” of 0.73 and were drawn at thedrawing rate of a coagulated filament of 5.0 m/min to prepare coagulatedfilaments. Then, the coagulated filaments were immersed into the secondcoagulation bath having the same composition at the same temperature asthe first coagulation bath and was subject to stretching by 1.6 times inthe bath. While washing with water, the filaments were further stretchedby 3.0 times and in hot water by 1.67 times. Then, the filaments wereoiled, dried on a hot roll at 150° C., crimped, heated and cut toprovide a staple fiber with a monofilament denier of 5.5 dtex. Theresults are shown in Table 1.

EXAMPLE 8

An acrylic fiber was prepared as described in Example 7, except thatcoagulated filaments were discharged into the first coagulation bathunder the condition of a ratio of “a drawing rate of a coagulatedfilament/a discharge linear velocity of a spinning feed solution from aspinneret capillary” of 0.98 and were drawn at the drawing rate of acoagulated filament of 6.0 m/min to prepare coagulated filaments, andwere then stretched by 1.2 times in the second coagulation bath havingthe same composition at the same temperature as the first coagulationbath. The results are shown in Table 1.

EXAMPLE 9

A monomer composition consisting of 92 wt % of acrylonitrile and 8 wt %of vinyl acetate was polymerized by aqueous suspension polymerizationusing ammonium persulfate-sodium hydrogen sulfite to prepare anacrylonitrile polymer with an average molecular weight of 130,000. Thepolymer was dissolved in dimethylacetamide to prepare a 24 wt % spinningfeed solution.

The spinning feed solution was discharged into the first coagulationbath from a spinneret with 6000 orifice holes. In the spinneret, aorifice hole 10 had an essentially Y-shape in which three branchedopenings 11 were radially extended from the enter as shown in FIG. 6 anda ratio A/B was 120 μm/40 μm (=3.0) wherein “A” and “B” are the lengthof each branched opening arm 11 from its center and the width of thebranched opening, respectively. The first coagulation bath consisted ofa 30 wt % aqueous dimethylacetamide solution at 40° C., and thecoagulated filaments were drawn from the first coagulation bath with adrawing rate 1.6 times of the discharge linear velocity of the spinningfeed solution.

Then, the coagulated filaments were immersed into the second coagulationbath consisting of a 30 wt % aqueous dimethylacetamide solution at 40°C. and was subject to stretching by 1.5 times in the bath. While washingwith water, the filaments were further stretched by 2.7 times and in hotwater by 1.9 times. Then, the filaments were oiled and dried on a hotroll at 150° C. The acrylic fiber thus obtained was crimped, heated andcut to provide a staple fiber with a Y-shaped cross section and with amonofilament thickness of 6.6 dtex.

A monofilament exhibited a Young's modulus of 6370 N/mm², and the staplefiber exhibited good luster and hand feeling.

A monofilament cross section was observed to determine a length from thefilament center to a flat arm tip “a” and the width of the arm “b”. Theratio of (length a)/(width b) was 5.0.

The acrylic fiber was subject to tension rupture and the rupture lateralsurface was observed. In the rupture lateral surface, a crack with alength of 200 μm extending along a fiber axis direction was observed inthe center of the fiber.

In the acrylic fiber in this example, the above crack had a length of200 μm and orientation was adequate in its surface as well as itsinside. The acrylic fiber was processed into a pile exhibiting good handfeeling with both softness and adequate flexibility because tips offilaments were fully split while their roots were not split.

EXAMPLE 10

A staple fiber with a Y-shaped cross section was prepared as describedin Example 9, except that an stretching ratio was 1.8 in the secondcoagulation bath. A monofilament obtained had a Young's modulus of 6900N/mm² and exhibited good luster and hand feeling.

A monofilament cross section and a monofilament tension rupture lateralsurface were observed as described in Example 9. A ratio of a/b was 4.0where “a” and “b” are a length from the filament center to a flat armtip and the width of the arm, respectively. In the tension rupturelateral surface, a crack with a length of 250 μm extending along a fiberaxis direction was observed in the center of the fiber.

The acrylic fiber of this example was processed into a pile exhibitingsoftness and adequate flexibility because tips of filaments were fullysplit while their roots were not split as was in Example 9.

COMPARATIVE EXAMPLE 1

The spinning feed solution described in Example 1 was discharged intothe first coagulation bath consisting of a 50 wt % aqueousdimethylacetamide solution at 40° C. using a spinneret with 40,000orifice holes and an orifice hole diameter of 60 μm to preparecoagulated filaments. The filaments were drawn from the firstcoagulation bath with a drawing rate 1.0 time of the discharge linearvelocity of the spinning feed solution. Then, while washing with water,the filaments were stretched by 2.7 times and in hot water by 1.9 times.Then, the filaments were oiled, dried on a hot roll at 150° C., crimped,heated and cut to provide a staple fiber with a monofilament denier of3.3 dtex.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.4 μm. Themonofilament exhibited a dry strength of 2.4 cN/dtex, a dry elongationof 45%. However, luster and hand feeling of the staple fiber was poor.

The fiber cross section was substantially an ellipse with a long/shortaxis ratio of 1.8. In the tension rupture lateral surface, there wereobserved no cracks 20 μm or longer extending along a fiber axis.

COMPARATIVE EXAMPLE 2

A staple fiber with a thickness of 3.3 dtex was prepared as described inComparative Example 1, except that dry heat stretching by 1.2 times wasconducted after hot water stretching.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.4 μm. Themonofilament exhibited a dry strength of 3.2 cN/dtex and a dryelongation of 30%.

The fiber cross section was a broad-bean shape with a long/short axisratio of 1.8. In the tension rupture lateral surface, there wereobserved no cracks 20 tm or longer extending along a fiber axis.

COMPARATIVE EXAMPLE 3

Preparation of a staple fiber was attempted as described in Example 3,except that filaments were drawn from the first coagulation bath with adrawing rate 1.2 time of the discharge linear velocity of the spinningfeed solution, but spinning was unstable due to considerable filamentbreaking in the first coagulation bath.

COMPARATIVE EXAMPLE 4

The spinning feed solution described in Example 1 was discharged intothe first coagulation bath consisting of a 67 wt % aqueousdimethylacetamide solution at 40° C. through a spinneret with 40,000orifice holes and an orifice hole diameter of 60 μm to preparecoagulated filaments. The filaments were drawn from the firstcoagulation bath with a drawing rate 0.8 time of the discharge linearvelocity of the spinning feed solution. Then, they were subject to dryheat stretching in the air, but the stretching was quite unstable due toconsiderable filament breaking.

COMPARATIVE EXAMPLE 5

The spinning feed solution described in Example 1 was discharged intothe first coagulation bath consisting of a 50 wt % aqueousdimethylacetamide solution at 40° C. using a spinneret with 40,000orifice holes and an orifice hole diameter of 60 μm to preparecoagulated filaments. The filaments were drawn from the firstcoagulation bath with a drawing rate 0.9 time of the discharge linearvelocity of the spinning feed solution. Then, the coagulated filamentswere immersed into the second coagulation bath consisting of a 50 wt %aqueous dimethylacetamide solution at 40° C. and was subject tostretching by 1.05 times in the bath. While washing with water, thefilaments were stretched by 2.7 times and in hot water by 1.9 times.Then, the filaments were oiled, dried on a hot roll at 150° C., crimped,heated and cut to provide a staple fiber with a monofilament denier of3.3 dtex.

In the above process, the thickness of the skin layer in a coagulatedfilament drawn from the first coagulation bath was 0.3 μm. Themonofilament exhibited a dry strength of 2.5 cN/dtex and a dryelongation of 45%.

The fiber cross section was substantially a broad-bean shape with along/short axis ratio of 1.8. In the tension rupture lateral surface,there were observed no cracks 20 μm or longer extending along a fiberaxis.

The staple fiber exhibited inadequate elasticity, and gave a cloth withpoor repulsion which did not have hand feeling required for a garmentsuch as a sweater or a home furnishing material such as a pile.

COMPARATIVE EXAMPLE 6

An acrylic fiber was prepared as described in Example 7, except thatcoagulated filaments were drawn at 8.0 m/min under the condition of aratio of “a drawing rate of a coagulated filament in the firstcoagulation bath/a discharge linear velocity of a spinning feed solutionfrom a spinneret capillary” of 1.18, the second coagulation bath was notused, and while washing with water, the filaments were stretched by 3.0times and 1.64 times in hot water. The results are shown in Table 1.

COMPARATIVE EXAMPLE 7

An acrylic fiber was prepared as described in Example 7, except thatcoagulated filaments were drawn at 10.0 m/min under the condition of aratio of “a drawing rate of a coagulated filament in the firstcoagulation bath/a discharge linear velocity of a spinning feed solutionfrom a spinneret capillary” of 1.47, the second coagulation bath was notused, and while washing with water, the filaments were stretched by 3.0times and 1.33 times in hot water. The results are shown in Table 1.

COMPARATIVE EXAMPLE 8

An acrylic fiber was prepared as described in Comparative Example 6,except that TiO₂was added to the spinning feed solution to 0.5% based onthe polymer. The results are shown in Table 1.

COMPARATIVE EXAMPLE 9

An acrylic fiber was prepared as described in Example 7, except thatcoagulated filaments were drawn at 4.0 m/min under the condition of aratio of “a drawing rate of a coagulated filament in the firstcoagulation bath/a discharge linear velocity of a spinning feed solutionfrom a spinneret capillary” of 0.59 and then the filaments werestretched by 2.0 times in the second coagulation bath at the sametemperature with the same concentration as the first coagulation bath.The results are shown in Table 1.

COMPARATIVE EXAMPLE 10

An acrylic fiber was prepared as described in Example 7, except thatcoagulated filaments were drawn at 11.4 m/min under the condition of aratio of “a drawing rate of a coagulated filament in the firstcoagulation bath/a discharge linear velocity of a spinning feed solutionfrom a spinneret capillary” of 1.68, the filaments were stretched by 1.5times in the second coagulation bath at the same temperature with thesame concentration as the first coagulation bath, and while washing withwater, the filaments were stretched by 2.0 times and 1.16 times in hotwater. The results are shown in Table 1.

COMPARATIVE EXAMPLE 11

The spinning feed solution in Example 9 was discharged in the firstcoagulation bath in Example 9 using the spinneret in Example 9. Thecoagulated filaments were drawn with a drawing rate 1.6 times of thedischarge linear velocity of the spinning feed solution and withoutconducting stretching in the second coagulation bath, while washing withwater, the filaments were stretched by 2.7 times and in hot water by 1.9times. As described in Example 9, the filaments were oiled and dried ona hot roll at 150° C. The acrylic fiber thus obtained was crimped,heated and cut to provide a staple fiber with a Y-shaped cross sectionand with a monofilament denier of 6.6 dtex.

A monofilament obtained exhibited a Young's modulus as low as 5400N/mm², and had poor repulsion.

A monofilament cross section and a monofilament tension rupture lateralsurface were observed as described in Example 9. A ratio of a/b was 6.0where “a” and “b” were a length from the filament center to a flat armtip and the width of the arm, respectively. In the tension rupturelateral surface, there was observed a crack extending along the fiberaxis in the center, but it was as short as 150 μm.

The acrylic fiber was processed into a pile, in which filament tips werenot adequately split and which was not soft because the above cracklength 150 μm was too short to give a fiber not fully oriented to itsinside. Furthermore, due to a Young's modulus as low as 5400 N/mm², thepile exhibited inadequate repulsion and poor flexibility.

TABLE 1 Total Maximum level Fiber bundle Color- draw difference Averagetilt surface Brushing developing R* ratio (μm) angle (°) lusterinesseffect property Ex.7 0.73 8.0 0.3 19 14.0 ◯ ◯ Ex.8 0.98 6.0 0.2 16 16.0◯ ◯ Comp 1.18 5.0 0.12 14 23.0 X ◯ Ex.6 Comp 1.47 4.0 0.08 12 26.0 X ◯Ex.7 Comp 1.18 5.0 0.2 15 9.0 ◯ X Ex.8 Comp 0.59 9.0 0.4 20 12.0 X ◯Ex.9 Comp 1.68 3.5 0.3 30 20.0 X ◯ Ex.10 *Ratio of drawingrate/discharge linear velocity of a spinning feed solution from a nozzle◯: Satisfactory X: Poor

Next, some of acrylic fibers obtained above examples and comparativeexamples were observed by scanning electron microscope (SEM). The imagesof SEM are shown in FIGS. 8 to 15.

Oblique view of the fiber obtained in example 1 is shown in FIG. 8(a). Alateral surface of the fiber ruptured in the tension test is shown FIG.8(b). Cracks with lengths of 20 μm or longer along the fiber axisdirection were observed in the tension rupture lateral surface.

Oblique view of the fiber obtained in comparative example 1 is shown inFIG. 9(a). A lateral surface of the fiber ruptured in the tension testis shown FIG. 9(b). It is found only short cracks along the fiber axisdirection were observed in the tension rupture lateral surface.

Oblique view of the fiber obtained in example 3 is shown in FIG. 10. Asshown in this figure, the fibers with round shape in the filament crosssection were obtained.

Oblique view of the fiber obtained in comparative example 5 is shown inFIG. 11. As shown in this figure, the fibers obtained in thiscomparative example have the cross section with a broad-bean shape incomparison with that obtained example 3.

Oblique view of the fiber obtained in example 7 is shown in FIG. 12(a).It is found that the flat shaped fibers were obtained in this example.As shown FIG. 12(b), on the surface of the fiber, corrugations withlarge level difference were observed.

Oblique view of the fiber obtained in comparative example 6 is shown inFIG. 13(a). It is found that the flat fibers were obtained in thiscomparative example as in example 7. As shown FIG. 13(b), unlike example7, the level difference of corrugations on the surface of the fiber isshort and the surface were smooth.

Oblique view of the fiber obtained in example 9 is shown in FIG. 14(a).It is found that the fibers with Y shape cross section were obtained inthis example. Cracks with lengths of 200 μm or longer along the fiberaxis direction were observed in the tension rupture lateral surface asshown FIG. 14(b).

Oblique view of the fiber obtained in comparative example 11 is shown inFIG. 15(a). It is found that the fibers with Y shape cross section wereobtained in this example as in example 9. As shown FIG. 15(b), unlikeexample 9, it is found that only short cracks along the fiber axisdirection were observed in the tension rupture lateral surface.

INDUSTRIAL APPLICABILITY

In conclusion, an acrylic fiber according to this invention has evenorientation in its surface and inside; is significantly improved in drystrength, dry elongation and dyeability; exhibits wool-like handfeeling; and is therefore quite suitable as a synthetic fiber forvarious applications such as a garment material, e.g., a sweater and ahome furnishing material such as a pile.

According to a process for manufacturing an acrylic fiber of thisinvention, the thickness of a skin layer in a coagulated filament iscontrolled to give a filament evenly coagulated to its inside.Specifically, inadequate diffusion of a solvent in the filament insideis avoided to prevent the solvent from being rapidly diffused duringwashing to make orientation even in the surface and the inside. Thus, anacrylic fiber significantly improved in dry strength, dry elongation anddyeability can be readily and exactly manufactured.

What is claimed is:
 1. An acrylic fiber (a) consisting of anacrylonitrile polymer comprising an acrylonitrile unit in at least 80 wt% and less than 95 wt %, (b) having a monofilament dry strength of 2.5to 4.0 cN/dtex, (c) having a monofilament dry elongation of 35 to 50%,and (d) forming a crack with a length of 20 μm or more in its tensionrupture lateral surface along the filament axis direction when rupturingthe monofilament in a tension test.
 2. The acrylic fiber as claimed inclaim 1 where a long/short axis ratio in the fiber cross section is 1.0to 2.0.